Sixth period

Before it was known that elements beyond uranium were capable of existence, the heaviest known natural elements, thorium, protactinium and uranium, were placed in a sixth period of the periodic classification, corresponding to the elements hafnium, tantalum and tungsten in the preceding period. It was therefore implied that these elements were the beginning of a new, fourth transition series, with filling of the penultimate n = 6 level (just as the penultimate = 5... 

The transition metals, in the center of the periodic table, fill d sublevels. Remember that a d sublevel can hold ten electrons. In the fourth period, the ten elements Sc (Z = 21) through Zn (Z = 30) fill the 3d subleveL In the fifth period, the 4d sublevel is filled by the elements Y (Z = 39) through Cd (Z = 48). The ten transition metals in the sixth period fill the 5d subleveL Elements 103 to 112 in the seventh period are believed to be filling the 6d subleveL... 

If the seventh period of the periodic table is 32 members long, it will be the same length as the sixth period. Elements in the same family will have atomic numbers 32 units higher. The noble gas following radon will have atomic number = 86 + 32 = 118. The alkali metal following francium will have atomic number = 87+32 = 119. 

According to a theory by Pauling, other stable electron configurations can be formed from 3d, 4s and 4p orbitals in the fourth period, and, similarly, of 4d, 5s and 5p in the fifth and 5d, fix and 6p electrons in the sixth period. There is, first of all, a configuration in which eight electrons are involved, 2d, 2s and 4p electrons that form four stable bonds. This configuration is called... 

In the sixth period is a subset of 14 metallic elements (numbers 58 to 71) that are quite unlike any of the other transition metals. A similar subset (numbers 90 to 103) is found in the seventh period. These two subsets are the inner transition metals. Inserting the inner transition metals into the main body of the periodic table, as in Figure 2.30, results in a long and cumbersome table. So that the table can fit nicely on a standard paper size, these elements are commonly placed below the main body of the table, as shown in Figure 2.31. 

The sixth-period inner transition metals are called the lanthanides because they fall after lanthanum, La. Because of their similar physical and chemical properties, they tend to occur mixed together in the same locations in the earth. Also because of their similarities, lanthanides are unusually difficult to purify. Recently, the commercial use of lanthanides has increased. Several lanthanide elements, for example, are used in the fabrication of the light-emitting diodes (LEDs) of laptop computer monitors. 

The typical display of the inner transition metals. The count of elements in the sixth period goes from lanthanum (La, 57) to cerium (Ce, 58) on through to lutetium (Lu, 71) and then back to hafnium (Hf, 72). A similar jump is made in the seventh period. 

Lanthanides Any sixth-period inner transition metal. Actinides Any seventh-period inner transition metal. 

The sixth and the seventh periods are the long periods. The sixth period includes 32 elements, and only 28 elements of the seventh period elements are known. 

The sixth period starts by filling of 6s orbital with cesium (Cs) and barium (Ba).The 4f orbitals start to be occupied after one electron goes to the 5d orbitals. 

In the fifth period of the Periodic Table, we find transition metal elements with configurations 4d 5s1 and 4d 5s°. For the corresponding elements in the sixth period, the configurations become 5d -16s2 and 5d 16s1, respectively. This change is due to the aforementioned stabilization of the 6s orbital caused by the relativistic effects. Table 2.4.4 lists the configuration of the elements concerned. 

Table 2.4.4. The ground electronic configuration of some elements of the fifth and sixth periods...

The properties of the elements of the sixth period are influenced by lanthanide contraction a gradual decrease of the atomic radius with increasing atomic number from La to Lu. The elements of groups 5 to 11 for the fifth and sixth periods have comparable stmctural parameters. For instance, Nb and Ta, as well as the pair Mo and W, have very similar ionic radii, when they have the same oxidation number. As a result, it is very difficult to separate Nb and Ta, and it is also not easy to separate Mo and W. Similarly, Ag and Au have nearly the same atomic radius, 144 pm. Recent studies of the coordination compounds of Ag(I) and Au(I) indicate that the covalent radius of Au is even shorter than that of Ag by about 8 pm. In elementary textbooks the phenomenon of lanthanide contraction is attributed to incomplete shielding of the nucleus by the diffuse 4f inner subshell. Recent theoretical calculations conclude that lanthanide contraction is the result of both the shielding effect of the 4f electrons and relativistic effects, with the latter making about 30% contribution. 

The variation of the melting points of the transition metals, as well as those of the alkali metals and alkali earth metals of the same period, are displayed in Fig. 2.4.6. It is seen that the uppermost curve (that for the elements of the sixth period) starts from Cs, increasing steadily and reaching a maximum at W. Beyond W, the curve starts to decrease and reach the minimum at Hg at the end. It is believed that this trend is the result of the relativistic effects. 

All the elements in any horizontal row of the periodic table are said to be in the same period. There are seven periods, the first consisting of just two elements. The second and third periods contain 8 elements each, and the next two contain 18 elements each. The sixth period has 32 elements (including 14 inner transition elements numbered 57 through 71, located at the bottom of the table), and the last period is not yet complete. The periods are conventionally numbered with the Arabic numerals 1 through 7 (Figure 1.6). 

Fortunately the chemical formulas can be rationalized on the basis of the size of the central atoms involved it is not necessary to memorize their formulas In general, second-period atoms are hmited to a maximum total coordination number (the total coordination number counts unshared electron pairs in the p-block) of fom third and fourth-period atoms can have maximum total coordination numbers of six fifth and sixth-period atoms can exceed a total coordination number of six. These observations explain the hydrolytic inertness (and persistence in the atmosphere) of compounds such as CF4 and SFe, which contain very strongly acidic cations they also explain why the formulas of fluoro anions vary (e.g. BF4 in period 2 AlFe in period 3 WFg in period 6). Evidently, because of the influence of Jt bonding to oxygen, central atoms in 0x0 anions fail to exhibit these coordination numbers, but instead settle for lower penultimate total coordination numbers 3 in the second period, for example, COs 4 in the third and fourth periods, for example, 8104 and 0004 6 in the fifth and sixth periods. 

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